In recent years, the demand for terrestrial mobile communications including cellular phones and portable phones has increased markedly. As a consequence, the need has increased for improved technology which more effectively utilizes the limited frequency bands to secure larger subscriber capacities. One type of system which has been offered for more effective frequency utilization is the code division multiple access system (CDMA). The CDMA system relies on spread spectrum transmission and provides improved reception over earlier transmission methods through the modulation of information signals by spread codes which have extremely low cross correlation while also having very sharp auto-correlation characteristics. An example of a terrestrial mobile communication system which utilizes the CDMA system for transmission is described in U.S. Pat. No. 4,901,307.
In the United States, existing CDMA transmission systems utilize a modulation method known as a direct spread system which permits a receiver known as a RAKE type receiver to separately demodulate and combine a plurality of detected multi-path components of a signal. An example of a prior art RAKE type receiver is described on pages 328 to 353 of IEEE. Proceeding Vol. 68, No. 3 (March 1980).
By way of background to the description of the invention to follow, a prior art spread spectrum communication system which uses the direct spread system will now be described. FIG. 1a shows the basic construction of a prior art spread spectrum transmitter. As shown in FIG. 1a, transmission data 49 is input to an information modulation means 50 which is used to modulate the transmission data. Spread code generating means 51 generates a spread code for use in spreading the modulated transmission data. Spread spectrum modulation means 52 uses the generated spread code to output a modulated spread spectrum signal. A transmission antenna 53 connected thereto is then used to transmit the modulated signal.
FIG. 1b shows the basic construction of a spread spectrum receiver including a reception antenna 54, a spread spectrum demodulation means 56 connected thereto, a spread code acquisition means 55, and an information demodulation means 57. The spread code acquisition means 55 is used to generate the spread code at the same phase as the spread code that was used in the transmitter to modulate the detected signal. The spread spectrum demodulation means 56 is used to demodulate the detected signal in a process which is complementary to that used by the spread spectrum modulation means 52 in the transmitter (FIG. 1a). The information demodulation means 57 is used to further demodulate the output of spread spectrum demodulation means 56 to yield reception data 58.
Information modulation means 50 of transmitter (FIG. 1a) produces a narrowband information signal having sufficient bandwidth only to carry the transmission data 49. After modulation with the spread code, however, the resulting signal is enlarged many times in bandwidth compared to the original narrowband information signal. In the receiver (FIG. 1b) the spread spectrum demodulation means 56 converts the wideband signal back into a narrowband information signal by multiplying it with the same spread code generated at the same phase by spread code acquisition means 55 and then integrating the result.
The transmissions detected at receiver antenna 54 (FIG. 1b) contain interfering frequency components due to spurious frequency signals and ambient thermal noise (shown as spikes and raised flat spectrum interference components in FIG. 1b). Reception of the spread spectrum signal reduces these interference components by despreading the detected signals with a spread code having very small cross-correlation with respect to the interfering signals.
In a mobile communication environment, transmission on a channel frequently occurs along several different transmission paths, due to reflection, refraction, diffraction, and scattering of the transmitted signal, as illustrated in FIG. 2a. Such effects are commonly referred to as multi-path transmission. For example, in FIG. 2a, a base station 59 and a mobile station 60 are situated in proximity to a reflecting object 61 such as a building. Path 62 shows a direct path for a transmission arriving directly from base station 59. Path 63 shows an indirect path for the same transmission which arrives delayed after being reflected by building 61. FIG. 2b shows the respective correlation levels for each multi-path component which are detected at different reception timings with respect to the direct transmission path 62 and the delayed transmission path 63, respectively.
In order to correctly demodulate a spread spectrum signal having different reception timings according to different multi-path components, a spread spectrum demodulation means 56 of a receiver must be assigned to demodulate the multi-path component at the correct reception timing.
Signals which are subject to multi-path transmission due to reflections caused by buildings and other objects are subject to location dependent destructive interference between the different multi-path components. RAKE type receivers, which have a plurality of spread spectrum demodulation means 56 can be used to compensate for such multi-path transmission by having separate demodulation means 56 for demodulating different multi-path components.
The operations of a conventional RAKE type spread spectrum receiver will now be described. A block and schematic diagram of a conventional RAKE type demodulator is shown in FIG. 3. As illustrated in FIG. 3, a received input signal 1 from an antenna is applied to the several spread spectrum demodulation means 2, 3, 4, and 5. Spread spectrum demodulation means 2 through 5 are each assigned a different reception timing to separately demodulate the multi-path components of a transmission signal which has been received along different transmission paths. The outputs 6 through 9 of spread spectrum demodulation means 2 through 5 are applied to a received signal combining means 10 which combines them as a weighted sum to yield a maximal-ratio combined signal.
The spread spectrum demodulator incorporates a correlation level search means 12 for determining a correlation level 13 for each reception timing of a signal according to its different multi-path components. The correlation level 13 for each reception timing is input to a phase assignment means 14 which sets the reception timings of the spread spectrum demodulation means 2 through 5 for use in demodulating the detected multiple multi-path components.
FIG. 4 illustrates an example of the correlation levels for multi-path components of a transmission signal. Specifically, FIG. 4 illustrates the correlation levels 16 through 20 of multi-path components which are detected at the respective reception timings to through t4. As indicated by FIG. 4, the correlation levels 16 through 20 are at a maximum for the reception timings t0 to t4 for each of the respective multi-path components.
Assignment of the reception timings for demodulation by the spread spectrum demodulation means 56 is performed so as to select the subset of reception timings at which the highest correlation levels are observed. Thus, in this example, the spread spectrum demodulation means will be assigned to demodulate at reception timings t0, t1, t2 and t4 at which correlation levels 16, 17, 18 and 20 are detected, respectively. Reception timing t3 at which the lowest correlation level 19 is observed, will not be selected for assignment to the spread spectrum demodulation means because such would result in decreased demodulation performance.
In a mobile communication environment, Rayleigh fading, and other phenomena cause large time-dependent variations in the correlation levels of the signals received along particular transmission paths. Rayleigh fading is a periodic phenomenon which varies with time at a particular location in proportional relation with the speed at which a mobile station moves. The correlation level for each multi-path component subject to such fading can vary independently by more than 20 dB. As a result, the correlation level search means 12 must continually track the correlation levels detected for each multi-path component of a signal.
Notwithstanding such fading, a system and method is need which will permit a phase assignment means of a spread spectrum RAKE type receiver to assign reception timings for demodulating different multi-path components signals by the spread spectrum demodulation means 2 through 5 which will always correspond to the group of detected multiple multi-path components having the highest correlation level.
However, the conventional RAKE type spread spectrum demodulator is not capable of always assigning reception timings for demodulation which correspond to the highest overall correlation level because the correlation level for each multi-path component is constantly changing as a consequence of the mobile station's movement. In addition, the phase assignment means 14 of the conventional RAKE type receiver is subject to control delays occasioned by the detecting operations performed by correlation level search means 12 and additional delays in the phase assignment means in changing the reception timing settings of the spread spectrum demodulation means 2 through 5. As a result, the prior art RAKE type receiver frequently does not operate at the maximum correlation level during movement of a mobile station and consequently does not provide optimum reception quality.
With reference to FIG. 5a, a phase assignment operation by the prior art RAKE type receiver will be described in which the time change in correlation level of a multi-path component is slow in comparison with the control response speed of the phase assignment means 14. With reference to FIG. 5b, a phase assignment operation by the prior art RAKE type receiver will be described in which the time change in the correlation level for a multi-path component is fast in comparison with the control response speed of the phase assignment means 14. For the sake of simplicity, the case will be considered in which the number of detected multi-path components is two and the receiver contains only one spread spectrum demodulation means.
FIG. 5a illustrates an example in which the time change in correlation level for a multi-path component of a communication is slower than the speed of control effected by the phase assignment means 14. Curve 30 shows a time change in the correlation level for a multi-path component A of a multi-path signal. Curve 31 shows a time change in correlation level of a multi-path component B of a multi-path signal. Time point 33 indicates the time at which the correlation levels for multi-path components A and B cross, such that path B has higher correlation level thereafter. Time point 34 indicates the time at which the spread spectrum demodulation means is switched from demodulating at the reception timing of multi-path component A to that of multi-path component B. Thus, the control delay for effecting a change in reception timing assignment is shown by interval 32. Interval 35 indicates when the phase of the spread spectrum demodulation means is set to that of multi-path component A, and interval 36 indicates when the phase of the spread spectrum demodulation means is set to that of multi-path component B. Thus, the phase assignment means 14 is capable of switching the reception timing assignment of the demodulation means to a reception timing for a different multi-path component at which the higher correlation level is detected, after a control delay 32.
FIG. 5b illustrates an example in which the time change in correlation level for a multi-path component is faster than the speed of control effected by phase assignment means 14. Curve 37 shows a time change in the correlation level for a multi-path component C of a multi-path signal. Curve 38 shows a time change in the correlation level for a multi-path component D of the multi-path signal. Intervals 39 indicate when the reception timing of the spread spectrum demodulation means is set to that of multi-path component C, and intervals 40 indicate when the reception timing of the spread spectrum demodulation means is set to that of multi-path component D. In this example, due to the slowness of the control response effected by the phase assignment means 14, in comparison with the time change in correlation levels, the prior art RAKE type receiver is incapable of performing a reception timing assignment which results in demodulation of the higher correlation level signal.
According to the existing system operation as shown in FIG. 5a, despite the time change in correlation levels for the multi-path components being slow compared to the control response speed of the phase assignment means 14, the reception quality deteriorates during the interval of the control delay 32 in which demodulation is performed at the reception timing for the lower signal correlation level. However, according to the existing system operation as shown in FIG. 5b, when the time change in correlation level of a multi-path component is faster than the control response speed of the phase assignment means 14, the reception timing assignment results in demodulation of a multi-path component which is lower in correlation level at a given point in time than the multi-path component C, which also has the higher average signal correlation level. In such case, as illustrated in FIG. 5b, it would be desirable to avoid frequently switching the reception timing assignment of the spread spectrum demodulation means and to assign a reception timing which reflects the higher average signal correlation level.
Although the above examples have illustrated, for simplicity, cases in which the number of multi-path components is two, and the receiver includes only one spread spectrum demodulation means, the skilled person in the art will understand its applicability to cases where a number of spread spectrum demodulation means, as in the prior art RAKE type receiver, are used to separately demodulate a number of multi-path components to be combined as a maximal-ratio demodulated signal.
The present invention seeks to solve problems occasioned by the conventional phase assignment techniques of the prior art receiving systems such as the RAKE type receiver. Specifically, the present invention seeks to provide a system and method by which phase assignment control is performed which would permit a RAKE type receiver to assign reception timings for demodulation which more closely correspond to the multi-path components which have the higher correlation level at a given point in time. Through employment of such phase assignment control, the reception quality for multi-path transmissions will be improved.
Accordingly, it is an object of the present invention to provide a system and method which permits a RAKE type receiver to assign reception timings for demodulation which correspond more closely to the multi-path components which have higher correlation levels at a given point in time.
Another object of the invention is to provide a system and method which permits a mobile communication user to be informed of an estimated moving speed of the mobile communication receiver in relation to a transmitter.
Another object of the present invention is to estimate the rate of change of a detected correlation level and to perform the assignment of a reception timing based on that estimated rate of change.
Still another object of the present invention is to provide a predicted value of a correlation level of a multi-path component signal, and to perform the assignment of a reception timing based on that predicted value.
A still further object of the present invention is to provide a averaging means having a selected averaging interval for determining an average correlation level of a multi-path component of a transmission signal in accordance with that averaging interval, and to perform the assignment of a reception timing based thereon.